Fiber-reinforced composite materials formed from reinforcing fibers and matrix resins are lightweight and have excellent mechanical characteristics, and therefore, fiber-reinforced composite materials are widely used for aerospace applications (aircraft members and the like), automobile applications (automobile members), sports applications (bicycle members and the like), general industrial applications, and the like. A fiber-reinforced composite material is obtained by molding a prepreg for a fiber-reinforced composite material, the prepreg being an intermediate material.
A prepreg is a material formed from reinforcing fibers impregnated with a thermosetting resin or a thermoplastic resin. Regarding the resin for prepreg, thermosetting resins are mainly used from the viewpoint of the heat resistance and the like of the fiber-reinforced composite material. Among them, from the viewpoint of obtaining a fiber-reinforced composite material having excellent heat resistance, excellent elastic modulus, low cure shrinkage, excellent chemical resistance, and the like, epoxy resins are most favorably used. In particular, in connection with the use applications where heat resistance is required, such as aerospace applications and industrial applications, 180° C.-curable epoxy resins are favorably used.
However, general 180° C.-curable epoxy resins require heating at 180° C. for 2 hours or longer for curing. Therefore, there is a problem that the production cost for the fiber-reinforced composite material increases because: (i) the heating furnace used for the molding of a prepreg requires a sufficient heating capability; (ii) the molding time is prolonged; and (iii) subsidiary materials are also required to have heat resistance of the same degree.
Regarding a method for solving this problem, for example, a method of subjecting an epoxy resin composition to primary curing at a low temperature of 80° C. to 140° C., demolding the cured product, and then post-curing the cured product at a high temperature of 180° C. or higher, is known (see Patent Literature 1).
Furthermore, an epoxy resin composition that is rapidly curable within 30 minutes at 150° C. has also been suggested (see Patent Literature 2).